Why does the File Browser's 'filepath' property include the blend file name once the file has been saved?
Thanks to all for the input. I am answering my own question.

I wanted to have in my AddOn a way to choose a folder containing some input files. It turns out that it was very easy using bpy.props StringProperty as panel property:

import_path: StringProperty(
        name = "Path",
        description = "Path to the folder containing the files to import",
        default = "",
        subtype = 'DIR_PATH'
        )


The DIR_PATH (or FILE_PATH, if expecting files) subtype is presenting a file system browser and retrieve the relative path, one just have to convert it to a absolute path in the operator:

bpy.path.abspath(self.import_path)


see here